The present invention relates to magnetic linear scaling devices for measuring magnitude of relative displacement of objects such as work and machine tool and the like.
Magnetic linear scaling devices are well known. One example of the magnetic linear scaling devices includes a magnetic scale having a track on which magnetic scale data is recorded, and a magnetic head movable along the magnetic scale to detect the magnetic scale data. The magnetic linear scaling devices are often used in an environment containing coolant, dust, cutting oil, or other contaminating elements during operation of the machine tool. These contaminating elements cause the magnetic scale data to be damaged, resulting in deteriorating accuracy of the detection by the magnetic head. There have been made attempts to protect the magnetic scale data on the track of the magnetic scale from any damage caused by such contaminating elements. One of the attempts, for instance, is to attach a stainless steel thin film of 0.3 mm thickness to the magnetic scale by means of an adhesive double coated tape. The stainless steel thin film must be subject to a demagnetizing process in advance of the attachment to the magnetic scale to make it magnetically neutral. In addition, when attaching the stainless steel thin film of a reduced thickness, for example less than 0.1 mm, the attachment operation is inconvenient. In another case, the magnetic scale has been coated with a solid-state lubricant, for example molybdenum disulfide, which has a thickness of only several microns to several ten microns. The coating layer is however insufficient in thickness so that the coating layer is easily peeled off from the magnetic scale. On the other hand, the coating layer with a sufficient thickness leads to increase in a manufacturing cost.
An object of the present invention is to provide an improved magnetic linear scaling device capable of performing accurate measurement by fully protecting magnetic scale data of a magnetic scale from being damaged without a reduction of cost performance.